But despite the dearth of information, Israeli media overwhelmingly reported that Netanyahu is resisting Kerry's entreaties on the subject of Iran's nuclear program. And if private differences develop into an open disagreement, some observers fret that the progress of fragile peace negotiations, which resumed in July after being stalled for almost three years, could be at risk.

The prime minister seems unmoored by the new pace of events, in which Israel's main ally, the United States, is energetically pursuing a diplomatic avenue of engagement with the Islamic Republic that Israel considers an existential threat. Iranian leaders have repeatedly threatened to annihilate Israel.

Netanyahu has warned for years against tying the outcome of negotiations over the Israeli-Palestinian peace process to proposals concerning Iran's nuclear ambitions. But on Wednesday, he appeared to make the connection himself, saying via Twitter: "Our aspiration for peace is liable to be severely affected if #Iran succeeds in its aspiration."

While in Rome, Netanyahu had sounded even less compromising on Iran, saying that a "partial deal that leaves Iran with nuclear capabilities is a bad deal. No deal is better than a bad deal."

Israel's deputy foreign minister, Yuval Steinitz, played down any stumbling blocks in negotiations with the US, saying, "We generally see eye to eye with the Americans on the final objective, which is to prevent Iran from getting a nuclear weapon, but there are sometimes small differences over the way to do that."

Within Israel, many deride Netanyahu's insistence on addressing Iran's nuclear ambitions at the expense of all other foreign policy objectives.

In an interview late Thursday, professor Avraham Sela, a Hebrew University expert on Israel's diplomatic situation, said Netanyahu’s myopic focus on Iran has led to “behavior bordering on hysteria."

"The issue of Iran's nuclear ability is a serious issue, but a chance has to be given to this new diplomatic avenue," he said.

“What did he think? That within a week everything would be clear? Everybody knew it would take several months, and while results may be bad — there's a possibility it may work," Sela added.

Israel, the Middle East's sole if undeclared nuclear-armed power, wants Iran to meet four conditions before sanctions are eased: halting all uranium enrichment; removing all enriched uranium from its territory; closing its underground nuclear facility in Qom; and halting construction of a plutonium reactor.

A report issued Thursday by the Institute of Science and International Security stated that Iran's "breakout" ability — the time required for the country to develop weapons-grade uranium — is shorter than previously estimated.

"Iran has steadily expanded the number of IR-1 centrifuges installed at both its Fordow and Natanz gas centrifuge plants. … Additionally, it has started installing its more advanced centrifuge model, the IR-2m centrifuge, at the Natanz Fuel Enrichment Plant (FEP)," the report said.